For years coatings have been used to protect or change the appearance of various substrates. Three types of coatings most commonly available are paints, stains and clear coatings. A paint is any pigmented liquid, liquefiable or mastic composition designed for application to a substrate in a thin layer which is converted to an opaque solid film after application. Paints generally contain higher non-volatiles (NV) than stains. A stain is a transparent, semitransparent or opaque solution or suspension of coloring matter (dyes or pigments or both) in a vehicle, designed to color a surface without hiding it, or to color a material into which it is incorporated. Stains are generally lower in viscosity and pigment content than paints and tend to penetrate more into the substrate than paints. Clear coatings are generally colorless or near colorless transparent liquids which form transparent protective and/or decorative films upon application, although they may contain low concentration of colorless pigments.
All coating compositions contain a resinous or resin-forming constituent called the "binder" which can be a liquid such as a drying oil or a resin that can be converted to a solid gel in the final coating. In some instances, where the binder is either a solid or is too viscous to be applied as a fluid film, a volatile solvent or "thinner" is also added. This evaporates after a liquid film is deposited, and the evaporation causes solidification of the film into the desired coating.
Pigments added to coating compositions are insoluble powders of very fine particle size, as small as 0.1 micron and can be up to about 20 microns. The binder together with the pigment and other conventional coating additives make up the non-volatile (NV) or film-forming part of the liquid coating composition.
Coating compositions may be classified into two very broad groups: (1) aqueous types of solutions or emulsions, which use water as the primary liquid or continuous phase; and (2) non-aqueous types of solutions or suspensions, which use organic solvents, such as hydrocarbons or oxygenated compounds as the primary liquid or continuous phase. Many of the same pigments and other components used in coating compositions can be used in both the aqueous and non-aqueous types of coatings.
The non-aqueous type of coatings generally contain an oil-soluble alkyd resin either alone or in combination with certain other film-forming agents such as drying oils. The non-aqueous coatings are generally made by blending the alkyds and drying oils with colored pigments, extender pigments such as clays and silicates, in a volatile organic solvent. In addition, metallic soaps (driers), such as cobalt, calcium, manganese and the like naphthenates are added to speed the dry time of these coatings. Also added may be such additives as anti-settling compounds, dispersing agents, flatting agents, adhesion promoters, water-proofing agents, mildewcides, and film preservatives, among others.
These non-aqueous coatings are characterized by good wetting penetration and adhesion to the substrate. The long term weathering of these coatings, however, often results in the occurance of one or more of the following problems: embrittlement, cracking, blistering, fading and chalking. Additionally, these non-aqueous solutions are flammable and present air pollution problems because of the volatile organic compounds used as solvents.
The aqueous type of coatings is characterized by the use of emulsion polymers as the binders for these products. The polymers are made by the emulsion polymerization of such compounds as vinyl acetate, methyl methacrylate, ethyl acrylates, styrene-butadiene, etc. These emulsion polymers may be blended with pigments, extender pigments, and water to make coatings. Also added are such items as freeze-thaw agents, coalescing agents, dispersing agents, mildewcides, thickeners, etc. These aqueous coatings exhibit excellent weathering characteristics because of the high molecular weight associated with the polymers used. Because of the water content, there is less VOC emission than with non-aqueous products. However, since the polymer film forms by coalescing, these aqueous products tend not to penetrate nor bond as well as their non-aqueous counterparts. They are furthermore subject to such phenomena as poor adhesion, a phenomenon known as tissue paper peeling and will raise the grain of wood.
In recent years there have been many attempts to overcome the shortcomings of the above-mentioned non-aqueous solution and emulsion type of coatings. One method has been to modify non-aqueous solution resins so that they become water soluble or water dispersable so as to avoid solvent emission and flammability problems and still yield good penetration and bonding to the substrate. Another method has been to emulsify such items as alkyd resins or drying oils into a latex emulsion such as acrylic latex resins, so as to improve bonding to and adhesion to the substrate, yet leave a high molecular weight film on the surface. In both cases, the resultant coatings suffer one or more detriments, such as poor storage stability, slow drying, difficulty in manufacture, and loss of long term weathering. The basic approach in both these systems has been to add, by some means, the resins to the water in such a manner that the water is the continuous phase.
It has now been found, however, that the non-aqueous solution and emulsion type of coatings may be modified by the addition of an aqueous latex emulsion into the non-aqueous coating composition so that the non-aqueous phase is continuous and the aqueous latex is dispersed or suspended therein. By using hydrated starch or a hydrated starch component as the dispersing agent, coating compositions can be made that will have the aqueous phase as the discontinuous phase dispersed throughout the non-aqueous continuous phase.